Junction circulator having a conductive septum in junction region

ABSTRACT

A junction circulator in which the usual magnetically biased ferrite post is modified by being separated from at least one conductive boundary by a dielectric gap and by having a conductive septum extending normal to the axis of the post at a point between the conductive boundaries. These modifications induce wave fields that more or less simulate those of the turnstile circulator and produce improved bandwidths of circulation.

United States Patent Clare Earl Barnes Bethlehem;

Brian Owen, Allentown, both of Pa.

Feb. 2, 1970 Nov. 2, 197 1 Bell Telephone Laboratories, Incorporated Murray Hill, NJ.

Inventors App]. No. Filed Patented Assignee JUNCTION CIRCULATOR HAVING A CONDUCTIVE SEPTUM IN JUNCTION REGION 6 Claims, 5 Drawing Figs.

U.S. Cl 333/11, 333/98 R int. Cl 1101p 1/32, HOlp 5/12 Field of Search 333/].1

DlELECTRiC GAP 27 [56] References Cited UNITED STATES PATENTS 3,350,663 10/1967 Siekanowicz et al. 333/1 .1 3,517,340 6/1970 Magalhaes 333/1 .1

Primary ExaminerI-lerman Karl Saalbach Assistant Examiner-Paul L. Gensler Attorneys-J2. J. Guenther and E. W, Adams, .lr.

ABSTRACT: A junction circulator in which the usual magnetically biased ferrite post is modified by being separated from at least one conductive boundary by a dielectric gap and by having a conductive septum extending normal to the axis of the post at a point between the conductive boundaries. These modifications induce wave fields that more or less simulate those of the turnstile circulator and produce improved bandwidths of circulation.

GYROMAGNE MATERIAL 5 5 PATENTEDNLWZ |97| $617,950

SHEET 1 0F 2 FIG. 1 PRIOR ART) FIG. 2

GYROMAGNETIC MATERIAL 25 DIELECTRIC GAP 27 C. EBARNE INVENTORS a OWEN 3 A T TOR/V5 V JUNCTION CIRCULATOR HAVING A CONDUCTIVE SEPTUM IN JUNCTION REGION BACKGROUND OF THE INVENTION This invention relates to symmetrical coupling devices for electromagnetic wave energy'and, more'particularly, to very broadband waveguide Y-junction circulators.

The basic Y-junctioncirculator comprises a conductively bounded junction of three waveguides having a magnetically biased gyromagsetic body extending along the axis of symmetry of the junction. Numerous variations of this basic structure, principally having to do with the size and shapeof the gyromagnetic body and with 'means for matching "its impedance to the remainder of the structure, have been proposed to improve one or another'of the operating characteristics of the circulator.

It is now clearly understood that circulator action'depends upon therelationship between the responses of-the junction to three modes of excitation, namely,an in-phase'modeand t'wo counterrotating modes, the reflection coefficients "of which must be mutually displaced inphase by I". Thedifferences in bandwidth of various forms of circulatorsdepend up'o'n'th'e degree to which it is possiblein a particular structure to maintain this phase relation as frequency ischanged.

One particular form has been referred to as a turnstile circulater, the name being descriptive of thestructural appendage which characterizes its construction. Specifically, the usual waveguide junction is supplemented by a shorted, circular waveguide stub rising out of one side of the junction in which the axially biased gyromagnetic body is located. The structure is of interest because of its ability to support and tune'the counterrotating modes in the circular guide stub relative to the phase of the in-phase mode in the junction.

Despite its advantages, the turnstile circulatorhas remained a laboratory curiosity because of its unwieldy and relatively complex mechanical structure. At least one attempt-to simplify the turnstile has been described by B. A. Auld in "The Synthesis of Symmetrical Waveguide Circulators, 7 IRE Transactions on Microwave Theory and Techniques, p. 238, Apr. 1959. However, this particularsimplification did not preserve the same waveguide modes in the junction and therefore did not preserve the bandwidth capabilities.

SUMMARY OF THE INVENTION In accordance with the present invention the electrical performance of the basic turnstile circulator is substantially improved upon with a structure that is no more mechanically complicated than a typical Y-junction circulator. More particularly, the usual magnetically biased gyromagnetic cylindrically shaped post extending along the axis of symmetry of the junction is foreshortened to create a dielectric discontinuity between one conductive boundary of the junction and one end of the post. At the same time a conductive septum normal to the axis effectively terminates the other end of the post at a point spaced from the other conductive boundary of the junction. In general, the dielectric gap induces counterrotating electric fields in the gyromagnetic body normal to the magnetic bias. These fields couple to axially propagating modes which are reflected by the septum back into the junction so that the gyromagnetic body acts as did the circular guide stub of the turnstile. The position of the septum provides a unique control of phase separation between the reflection coefficients as required for circulation. However, all parts of the structure are fully contained within the junction.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cutaway perspective view of a typical prior art turnstile circulator;

FIG. 2 is a cutaway perspective view of a junction circulator in accordance with the present invention;

FIG. 3 shows typical reflection coefficient characteristics useful in understanding the operation of the invention; and

FIGS. 4 and 5 show in cross section alternative arrangements of components within thejunction of FIG. 2.

2 DETAILED DESCRIPTION Referring more particularly to FIG. l,the prior art circulator is shown for the purpose of comparison with the present invention. It comprisesthree rectangular waveguideslli, and I2 intersecting in a Y'at angles of in 'anI-I-plane junction (theplane of the guides broad dimension) to form a conductively bounded common region from which the waveguide branches symmetrically extend. Extending coaxially with the axis of symmetry of 'the Yfrom the upper boundary of the common'regionis a section of circular waveguide I3 thatis coupledat its lowerend by a circular aperture to the junction and thatisclosed at its upper e'nd by a'shortin'g bouh'dary l4. A cylinder 15 of magnetically polarized gyromagnetic material, such as yttrium iron garnetor ferrite, is located axially within guide 13. CylinderlS is longitudinally biased 'alongthe axis of symmetry by being permanently magnetically polarised or polarizedby theuse of externalm'agn'etsas'schematically representedby the vector H Operation of such'a circulator is usually explainedby'dividing the' excitation'of one port'of the junctioninto three excitations eachinvolving excitation-of allthree' ports. The threeexcitations correspond to the 'eige'nvectors for the scattering matrix for the junctiomA first excitation excites all three ports equally and in phase while the remaining two excitations result inequalexcitations with'ph'ases that'resultin counterrotating circular polarizations within the junction. The requirement for circulationin terms of these'excitations is that their reflection coefficients corresponding to theeigenvalues for the scattering matrix be displaced in phase by 120.

It isus'eful to examine the fields at the axis of symmetry due' toeach of-these excitations. Forthe in-phase mode, the components of electric field parallel tothe'axis of symmetry due to the excitation of the three ports will be in phase and simply add to one another. The transverse components, while in phase, are space displaced by 120 and cancel vectorially. Therefore, the electric field at the axis of symmetry due to the in phase mode lies only along the axis of symmetry. Similarly,

for the counterrotating modes, the components of electric and sum to zero. The transverse components, while phase displaced'by 120, are spaced displaced by 120 resulting in circularly polarized fields. Similar arguments could be made about the magnetic fields with the conclus'ion that the counterrotating modes can, and the in-phase mode cannot, couple to waves travelling along the axis of symmetry. This provides the means for adjusting the 'reflectioncoefficients as required for circulation. i 7

Thus in the prior art structure of FIG. I the counterrotating modes propagate up the circular loaded guide 13 with an electric new and a transverse magnetic field normal to the biasing field li and are reflected back toward the junction by boundary 14. The net phase shifts for these modes with the gyromagnetic material unmagnetize'd are identical and are determined by the length of circular guide 13. Magnetizin'g cylinder 15, however, increases and decreases the path lengths of the counterrotating modes, respectively, and by adjusting H and the length of guide 13 and cylinder 15, these modes can be separated by 120 from each other and from the inphase mode as required for circulator action. This corresponds to the Faraday rotation by cylinder 15 of 60". The iriphase mode, on the other hand, cannot propagate iiito circular guide 13 but is resonantly supported within the junction, resonant frequency normally being remote from the operating frequency.

With this background in mind principles of the present invention may be understood from FIG. 2. In all cases in which the structure, materials or principles of operation are the same as those described above, a detailed descriptioii thereof need not be repeated. I j v I Referring'then to FIG. 2 a waveguide junction like that of FIG. I is shown comprising guides 20, 21 and 22 corresponding in every respect to guides 10, 11 and 12. No external appendage is required. Instead, gyromagnetic elemeht 25 takes the form of an axially biased cylinder located within the common region of the junction on the axis of symmetry. Gap 27 filled either by air, or by a suitable nonmagnetic dielectric material having dielectric constant close to that of air or at least substantially different from that of cylinder 25, forms a space between the lower end of cylinder 25 and the lower conductive boundary of the junction.

In accordance with the invention, the top end of cylinder 25 is terminated in a conductive and reflecting discontinuity formed by a thin surface 28 of conductive material such as copper, gold or aluminum foil bonded to the end surface and covering the full diameter of the end surface in a plane perpendicular to the axis of the cylinder. In accordance with a preferred embodiment the space between surface 28 and the top conductive boundary of the junction is filled by a second cylinder of gyromagnetic material 29 thus making surface 28 a conductive septum dividing one cylinder into two parts. Such a structure is preferably assembled as a laminate of two cylinders of gyromagnetic material bonded to either face of a conductive foil.

The significance of septum 28 as it defines the length of cylinder 25 as well as the significance of gap 27 can be understood when it is recalled that in an ordinary H-plane resonantjunction, the electric fields are everywhere parallel to the axis of symmetry. The region formed by gap 27, however, has a dielectric constant and permeability product that is different from that of the region occupied by gyromagnetic material of cylinder 25 so that the phase constants of the two regions differ. This creates an electric field in the plane of the interface between the two regions. Thus, only the counterrotating excitations launch waves as dielectrically supported modes in cylinder 25, travelling up cylinder 25 to be reflected at septum 28 and to couple back into the junction at gap 27.

The in-phase mode sees cylinders 25 and 29 together as a single dielectric resonator since this mode has no circularly polarized magnetic fields and does not excite any mode propagating along the axis of cylinders 25 and 29 and thus there is no gyromagnetic interaction with its material. Further, since the in-phase electric field is normal to septum 28, the mode is unaffected by the septum. Therefore, the diameters of cylinders 25 and 29 provide a means for controlling the phase of the in-phase mode while septum 28 provides the means for controlling the phase of the two counterrotating modes relative to the phase of the in-phase mode. The adjustments are independent since the separation between gap 27 and septum 28 exclusively determines the path length for the rotating modes.

The relationships can be seen from FIG. 3 which shows typical reflection coefficients in phase degrees of the several modes discussed above as they vary with frequency. Thus, the in-phase mode as represented by curve 31 (considered as having a resonant delay) is adjusted so that its most linear portion falls within the band of intended operation in a given junction by controlling the diameters of cylinders 25 and 29. With the cylinders unmagnetized, the spacing between septum 28 and gap 27 is selected so that the counterrotating modes, which together form a single linearly polarized mode as represented by curve 32 (considered as having linear delays) falls 180 away from curve 31. Increasing the spacing between septum 28 and gap 27 has the effect of increasing the phase separation between curves 31 and 32. l-I is then increased to separate the counterrotating modes by 120, raising one and lowering the other as indicated by curves 33 and 34, respectively. Circulation is then possible over the full range in which the curves generally parallel each other as indicated.

A typical embodiment according to these considerations would have the following illustrative proportions. Using a waveguide with a 2:1 aspect ratio and operating within the standard recommended frequency range for dominant mode propagation; the gyromagnetic cylinder would have a diameter of approximately one wavelength in the gyromagnetic medium at the lowest operating frequency; the separation between gap and septum would be typically of order onequarter wavelength in the gyromagnetic medium; the gap would be typically of the order of one-fifth of the waveguide height; and the gyromagnetic material would be selected to avoid low field losses in accordance with standard practice for low field devices.

It will be noted that while cylinder 29 above septum 28 serves only as dielectric material, it is preferable that this portion be formed from gyromagnetic material like that of cylinder 25 below septum 28 to simplify the construction, improve the magnetic biasing circuit, and minimize the dielectric discontinuity at septum 28. However, cylinder 29 can, if desired, be replaced by a nonmagnetic dielectric preferably having a dielectric constant near to that of the gyromagnetic material.

In certain cases it may be desirable to modify the coupling to the counterrotating modes as shown in FIG. 4 by employing two dielectric gaps. Thus, in the structure of FIG. 4 the counterrotating modes are generated at both dielectric gaps 41 and 42, propagate in opposite directions to be respectively reflected by septa 43 and 44 interposed equal distances from gaps 41 and 42, respectively. Septa 43 and 44 divide the gyromagnetic material into parts 45, 46 and 47 of which the gyromagnetic properties of only parts 45 and 47 are used.

The embodiment shown in FIG. 5 in effect reverses the relative positions of two dielectric discontinuities produced by a single gap 51 and the septa as compared to FIG. 4. Duplicate counterrotating modes are respectively generated at both interfaces between nonmagnetic dielectric 51 and gyromagnetic cylinders 54 and 55, propagate in opposite directions in cylinders 54 and 55 to be reflected by septa 52 and 53. While this structure bears superficial similarity to the one shown by Bowness in Us. Pat. No. 3,136,962, June 9, 1964, it is noted that the use of septa 52 and 53 afford optimum mode conversion dimensions for gap 51 and the optimum phase length for gyromagnetic cylinders 54 and 55 not otherwise possible in the prior art.

The present invention provides an improvement upon circulators of the turnstile type. While particularly illustrated by way of the three branch or Y-junction form, it should be noted that a four branch tumstile junction has been described by P. J. Allen in Us. Pat. No. 2,867,772, granted June 6, 1959, and in the [RE Transactions on Microwave Theory and Techniques, Oct. I956 on P. 223. The principles of the invention are equally applicable to improving this four branch form as will be obvious to one skilled in the art in view of the foregoing teachings.

What is claimed is:

l. A broadband circulator for electromagnetic wave energy comprising a conductively bounded structure having a plurality of branches symmetrically extending away from a conductively bounded common region having a pair of opposite conductive boundaries and adapted to support said wave energy with an electric field perpendicular to said boundaries and a magnetic field lying substantially in loops in planes parallel to said boundaries, a body of magnetically polarized gyromagnetic material disposed on the axis of symmetry of said common region, said body being spaced from at least one conductive boundary of said common region to leave a dielectric gap therebetween, and a conductive septum extending nonnal to said axis between the conductive boundaries of said common region.

2. A broadband circulator for electromagnetic wave energy comprising a conductively bounded structure having a plurality of branches symmetrically extending away from a conductively bounded common region having a pair of opposite conductive boundaries and adapted to support said wave energy with an electric field perpendicular to said boundaries and a magnetic field lying substantially in loops in planes parallel to said boundaries, at body of magnetically polarized gyromagnetic material having a longitudinal axis symmetrically disposed in said common region, means for creating a dielectric discontinuity at one end of said body, and means for creating a conductive and reflecting discontinuity at a point on said body located between said dielectric discontinuity and a conductive boundary of said common region.

5. The circulator of claim 4 wherein a second body of gyromagnetic material fills the space between said conductive member and a conductive boundary of said common region.

6. The circulator of claim 4 wherein dielectric material fills the spaces between said body and said member and both of the conductive boundaries of said junction.

* l i i 

1. A broadband circulator for electromagnetic wave energy comprising a conductively bounded structure having a plurality of branches symmetrically extending away from a conductively bounded common region having a pair of opposite conductive boundaries and adapted to support said wave energy with an electric field perpendicular to said boundaries and a magnetic field lying substantially in loops in planes parallel to said boundaries, a body of magnetically polarized gyromagnetic material disposed on the axis of symmetry of said common region, said body being spaced from at least one conductive boundary of said common region to leave a dielectric gap therebetween, and a conductive septum extending normal to said axis between the conductive boundaries of said common region.
 2. A broadband circulator for electromagnetic wave energy comprising a conductively bounded structure having a plurality of branches symmetrically extending away from a conductively bounded common region having a pair of opposite conductive boundaries and adapted to support said wave energy with an electric field perpendicular to said boundaries and a magnetic field lying substantially in loops in planes parallel to said boundaries, a body of magnetically polarized gyromagnetic material having a longitudinal axis symmetrically disposed in said common region, means for creating a dielectric discontinuity at one end of said body, and means for creating a conductive and reflecting discontinuity at a point on said body located between said dielectric discontinuity and a conductive boundary of said common region.
 3. The circulator of claim 1 wherein said conductively bounded structure comprises three rectangular waveguides forming a Y-junction.
 4. The circulator of claim 2 wherein the ends of said body are both spaced from the conductive boundaries of the junction such that the other end of said body coincides with said point, and wherein a thin conductive member is bonded to the surface of said other end.
 5. The circulator of claim 4 wherein a second body of gyromagnetic material fills the space between said conductive member and a conductive boundary of said common region.
 6. The circulator of claim 4 wherein dielectric material fills the spaces between said body and said member and both of the conductive boundaries of said junction. 